Breakdown of hypothalamic releasing hormones and related polypeptides will be studied with the purpose of delineating the enzymatic systems involved in their inactivation. Breakdown will be measured in different anatomical regions of brain and neurosecretory areas, and compared to rates of inactivation in blood and c.s.f. Split products will be detected by a microprocedures capable of separating amino acids, peptides and glycinamide. Studies on subcellar fractionation and extraction will be undertaken as a prelude to enzyme purification with emphasis placed on a neutral endopeptidase cleaving internal bonds of LH-RH (luteinizing hormone-releasing hormone), a pyroglutamyl peptidase cleaving TRH (thyrotrophin releasing hormone), aminopeptidases inactivating GH-RIH (somatostatin), and C-terminal cleaving enzymes liberating dipeptides of glycinamide from LH-RH, MIF (melanocyte inhibiting factor), and oxytocin. Studies on sites of cleavage will be supplemented by conventional end-group procedures utilizing DNP and fluorescamine. Classical methods similar to that used for other brain enzymes will be applied for purification supplemented in some cases by affinity chromatography, isoelectric focusing and preparative acrylamide electrophoresis. Structure-activity relationships will be studied in crude and purified systems using analogs of LH-RH substitued with D-amino acids at internal positions, and/or with ethylamide at the C-terminal: with 3-methyl His 2-TRH, and with N-blocked Cyst3 derivatives of GH-RIH. An attempt will be made to relate susceptibility to digestion with biological activity and thus provide a rational basis for predicating biological activity and the preparation of newer analogs with longer half-lives. The design of analogs capable of resisting degradation will aid in therapeutic applications.